Studies were continued on biochemical characterization of pneumococcal autolysin and protective immunity in mice to pneumococcal polysaccharide-protein conjugates. A number of pneumococcal cell-surface antigens are considered to be virulence factors. There are several protein antigens and toxins including pneumolysin, autolysin, pneumococcal surface protein A (PspA) and other minor antigens.Autolysin was isolated from pneumococcal R61 strain and purified. The autolysin exhibits the major enzymatic activity of N-acetylmuramic acid L-alanine amidase. It is involved in mediating the release of pneumolysin from pneumococcal cytoplasm. After purification, the specific activity of autolysin increased 143 fold. The yield was 0.9% of the crude extract. It exhibited a single band in SDS-PAGE, having a molecular weight of 36,000. Conjugation of pneumococcal type 9V PS or type 19F PS to a carrier protein may produce sufficient antibody for prevention of pneumococcal infection during childhood. Pneumolysin and autolysin may serve as effective carriers for PS-protein conjugate vaccines. Studies on bacterial clearance and antibody response were continued in young mice immunized with type 9V or type 19F PS-protein conjugates during early life to examine the protective immunity of young mice to pneumococcal infection. In other studies, a 19F PS-ply conjugate induced high 19F PS antibodies in young mice as measured by RIA . These antibodies induced rapid bacterial clearance of 19F cells from blood after challenge with 5 x 104 cfu/dose as compared to the non-immunized controls.